1. Field of the Invention
The present invention relates generally to data processing systems and more particularly to carry apparatus used when the system executes a decimal multiply instruction.
2. Description of the Prior Art
Numerous techniques for multiplying two numbers are known in the prior art. Some such techniques are described in the book entitled "Digital Computer Design Fundamentals" by Y. Chu, published by McGraw-Hill Book Company Inc., 1962.
U.S. Pat. No. 3,319,056 is typical of the technique wherein a decimal digit multiplicand and a decimal digit multiplier are processed by forming partial products. The partial products are formed by adding the multiplicand decimal digits to the partial product the number of times specified by each multiplier decimal digits. The multiplicand is shifted one decimal digit position relative to the partial product for each higher order multiplier decimal digit. A partial product decimal digit at a digit position is developed by adding the carry decimal digit generated from the lower order digit position to the partial product decimal digit on one cycle to generate a new partial product decimal digit, and by adding the multiplicand decimal digit to the new partial product decimal digit on a next cycle to generate the partial product decimal digit at this digit position. Also, a 4 bit counter is required to develop the carry for the next higher digit position of the partial product.
This approach presents the problem of requiring an additional carry cycle for each decimal digit in the multiplier, thereby increasing the time for executing the decimal multiply instruction. Also, the 4 bit carry counter with its associated control logic increases the cost and complexity of the system.
It should be understood that the references cited herein are those of which the applicants are aware and are presented to acquaint the reader with the level of skill in the art and may not be the closest reference to the invention. No representation is made that any search has been conducted by the applicants.